An increase in the power consumption of semiconductor devices has led to a need for semiconductor packages which have a lower thermal resistance. In general, the plastics and ceramics commonly used to package semiconductor devices do not dissipate heat sufficiently for some types of devices. Examples of devices requiring good heat dissipation include fast static RAMs (random access memories), gate arrays, and microprocessors. One of the most common and effective approaches to greater thermal dissipation in semiconductor devices is the addition of a heat sink, or heat spreader, to the package. A heat sink is typically made of a material having a high thermal conductivity, such as copper, and ideally would be in good thermal contact to the semiconductor die, would have a surface which is exposed to the ambient, and would have a maximized surface area.
Heat sinks used in semiconductor devices have taken many forms. Heat sinks may have irregular topographies, such as channels or grooves, to increase the exposed surface area of the heat sink, thereby improving thermal dissipation. In addition, a number of materials have been used as heat sink materials, including copper, copper alloys, copper-tungsten alloys, aluminum, aluminum alloys, molybdenum, and composites of these materials. Other features of heat sinks which have been used in the past are features which improve the adhesion of the heat sink to a semiconductor package. In the case of plastic semiconductor packages, using a heat sink may result in poor adhesion between the plastic encapsulating material and the heat sink. To overcome this problem, semiconductor manufacturers have used heat sinks having features such as dimples, holes, or roughened surfaces to achieve better adhesion.
Apart from achieving sufficient adhesion between a heat sink and a package body material, other problems of incorporating a heat sink into a semiconductor device exist. In the assembly of plastic semiconductor device packages, it is possible to incorporate a heat sink during the semiconductor die bonding operation, during the molding of the package body, or a heat sink may be attached after the package body is formed. However, a variety of manufacturing difficulties exist with each of these techniques.
To include a heat sink during a die bonding operation, a semiconductor die may be attached directly onto a heat sink which is attached to a leadframe, rather than onto a flag or mounting surface which is already part of the leadframe. While it seems reasonable to mount a die directly onto a heat sink in order to achieve good thermal conduction from the die to the heat sink, the added weight of the heat sink on a leadframe can cause substantial damage to the leadframe due to handling. Moreover, some of the equipment used in subsequent assembly operations may need to be modified in order to accommodate the presence of the heat sink on the leadframe.
To incorporate a heat sink within a device during the molding of a package body, the heat sink may be placed directly in a mold tool cavity such that it is at least partially encapsulated during the formation of the package body. Encapsulating the heat sink while forming the package body can have the advantage of achieving a standard package outline. Furthermore, it does not require the addition of an assembly operation to a conventional assembly process flow. A disadvantage associated with this method is that during the injection of an encapsulation material into the cavity, the heat sink is often moved, causing it to be misaligned within the package. There is also a risk that the heat sink will become completely encapsulated, thereby leaving no exposed surface from which to dissipate heat. Another disadvantage is that it is difficult to achieve good thermal contact between the heat sink and the semiconductor die. Air or the encapsulation material may be trapped between the heat sink and the die, thus increasing the overall thermal resistance of the device.
Incorporating a heat sink after package body is formed generally involves attaching a heat sink to the exterior of the package through the use of an adhesive material, such as a thermally conductive epoxy. However in doing so, the heat sink is positioned away from the die such that good thermal conduction away from a semiconductor die to the heat sink is obstructed by the package body material and any voids which might be present in the material. Along with the disadvantage of not having the heat sink in good thermal contact with the die, there is also a disadvantage of having a non-standard package outline. Furthermore, an attached heat sink increases the overall size of a semiconductor device which is an undesirable feature to most end users of semiconductor devices. Therefore a need existed for an improved thermally enhanced semiconductor device which can be readily manufactured with minimal modifications to conventional assembly equipment and which maintains the outline of a standard package.